In Briggs, U.S. Pat. No. 3,690,151, commonly assigned with the present invention, there is disclosed a method of and apparatus for detecting leaks of light gases, such as helium, from vacuum equipment wherein a high vacuum pump, such as a diffusion pump, is connected directly between the equipment and a gas monitoring instrument, such as a mass spectrometer. The equipment being monitored and the mass spectrometer are respectively connected to a foreline and an inlet of the diffusion pump. The diffusion pump effectively functions as a filter to enable a significant percentage of the light gas to flow from the equipment being monitored to the mass spectrometer, while virtually preventing the flow of heavy gases, such as water vapor and nitrogen in air, between the equipment and the mass spectrometer. Apparently, there is back diffusion of the light gases through vaporized jets of a diffusion pump fluid that is frequently an organic oil. The heavy gases, however, cannot back diffuse through the oil jet as easily. The mass spectrometer is responsive only to the light gas leaking from the equipment and relatively accurate indications of the amount of light gas leaking from the monitored equipment are obtained. Commercial equipment utilizing this principle has been extensively marketed under the trademark "Contraflow".
Diffusion pumps used in connection with leak detectors of the type disclosed by the Briggs patent ideally have a high tolerable forepressure for the heavy gases, while allowing the gas being detected to back diffuse quite readily from the foreline outlet of the diffusion pump to the diffusion pump inlet. As is well known, tolerable forepressure is the pressure at the diffusion pump foreline outlet which causes the vapor jet of the diffusion pump to collapse, allowing gas to pass almost freely from the foreline to the pump inlet. For virtually any foreline pressures less than the tolerable forepressure, the pressure at the inlet is maintained at a relatively constant, high vacuum, such as 10.sup.-6 torr. For foreline pressures greater than the tolerable forepressure, the inlet and foreline pressure are approximately equalized. Typical tolerable forepressures are on the order of 0.3 torr.
The tolerable forepressure of a diffusion pump is a direct function of the oil vapor density in an annular space or throat almost immediately downstream of a nozzle through which the oil vapor is diffused. If the cross-sectional area of the annular throat is reduced, less oil vapor is required to maintain the same oil density in the throat area. Decreasing the cross section of the throat, however, results in a decrease in pump speed. In diffusion pumps utilized in the normal operating mode, i.e., diffusion pumps that pump a load connected to the inlet to a high vacuum condition and which are not normally used in accordance with the teaching of the Briggs patent, the cross-sectional area of the throat is always an engineering compromise.
At foreline pressures lower than the tolerable forepressure and with the vapor jet fully formed, there is always a transfer of gas from the foreline to the pump inlet by a normal diffusion process. The amount of gas transferred from the foreline to the pump inlet is a function of the density of the oil vapor in the throat and the molecular weight of the gas flowing between the foreline and the pump inlet. The diffusion rate is lower for heavy gas molecules than for light gas molecules, which diffuse more quickly than heavy molecules.
The ratio of inlet pressure to the pressure at the foreline (P.sub.i /P.sub.f) for a given gas is so small in a well designed diffusion pump operating in the normal mode as to be of no practical significance. The inlet pressure (P.sub.i) can only be measured with a very sensitive instrument. In the diffusion pump which has generally been employed with the Contraflow leak detector, enough helium may back diffuse between the foreline and the pump inlet so as to be measured with a mass spectrometer leak detector. Hence, the diffusion pump which has generally been employed in connection with the Contraflow leak detector performs two functions: (1) permits some back diffusion of the light gas (generally helium) from the foreline to the mass spectrometer while restricting the back diffusion of air from the foreline to the mass spectrometer; and (2) maintains a high vacuum in the mass spectrometer.
As noted supra, a diffusion pump of the type preferably employed in a Briggs type leak detector ideally has a high tolerable forepressure for air, but should allow helium to back diffuse quite readily. Unfortunately, a high tolerable forepressure for air requires a high vapor density, while a high rate of back diffusion for light gases requires a low vapor density.
In the diffusion pump generally employed in the present commercial, Contraflow leak detector, the amount and density of the vapor supplied to the nozzle is increased or decreased by varying the amount of energy supplied to a heater for a pool of the diffusion pump fluid in order to change the ratio P.sub.i /P.sub.f. A decrease in power results in an increase in the ratio P.sub.i /P.sub.f. More helium diffuses back through the pump but unfortunately the tolerable forepressure decreases.